The therapy of disseminated cancer is a fundamental problem in clinical medicine. When a cancerous tumor is localized and accessible, it can be surgically removed or treated with radiation therapy. However, once a cancer has metastasized, the prognosis for longterm survival generally decreases markedly, and in addition, there is a marked decline in the general quality of life. When metastasis has occurred, systemic chemotherapy is generally the only therapeutic option available which can be used to prolong life. Although some exceptions exist, the existing drugs used for the systemic chemotherapy of metastasized cancer are palliative rather than curative, and their considerable toxicity results in marked morbidity for the patient.
Heretofore, there have been no known anticancer methods or drugs which involve a two-step method of the type described herein to achieve their active anticancer effect. In contrast, almost all of the known anticancer chemotherapeutic agents are administered in their active form or are metabolically transformed in vivo to their active forms in a nonspecific manner, and therefore all tissues, normal and neoplastic alike, are exposed to the active agent. This accounts for the well known toxicities associated with cancer chemotherapy. Thus, it would be advantageous to have a method of cancer chemotherapy which became active only after localization of the drug to the cancer had occurred, thereby possibly minimizing or reducing the toxic effects on normal tissues. In addition, almost all of the presently available anticancer drugs are considered to act at the level of the genetic apparatus of the cell (The Pharmacological Basis of Therapeutics, eighth ed., 1990. pp 1208-1263). It would therefore also be advantageous to have a method of cancer chemotherapy which acted at the cancer cell membrane instead of within the cell, thereby providing a method by which the internal cancer cell defenses and mechanisms of resistance against anticancer drugs might be bypassed, and thus provide a new or alternative treatment method to assist those patients whose malignancies have proven resistant to conventional anticancer therapies. Heretofore there have been no methods of cancer chemotherapy which achieve these advantages and therefore none lie within the scope of the present invention of a two-step method for the treatment of cancer.
It is known that certain metal ions and metallo-compounds such as copper, iron, and heme, can react with peroxides to generate powerful oxidant species. An example of this is the Fenton reaction in which the ferrous ion reduces H.sub.2 O.sub.2 to generate the hydroxyl free radical (OH.sup..cndot.), a known powerful oxidant, as per the following reaction (Walling, C., Fenton's Reagent Revisited, Accnts of Chemical Research 8:125; 1975): EQU Fe.sup.2 ++H.sub.2 O.sub.2 .fwdarw.Fe.sup.3 ++OH.sup..cndot. +OH.sup.-
The hydroxyl free radical is known to be an extremely powerful oxidant species, and is known to react with a wide variety of biological components (such as amino acids, proteins, lipids, carbohydrates, and nucleic acids) with very high reaction rate constants. The hydroxyl free radical can react with hydrogen peroxide to generate the perhydroxyl free radical (HO.sub.2.sup..cndot.). The perhydroxyl free radical can dissociate to generate the superoxide free radical (O.sub.2.sup. .cndot.), dismutate to generate H.sub.2 O.sub.2 and oxygen, and react with O.sub.2.sup. .cndot. to generate H.sub.2 O.sub.2 and oxygen. O.sub.2.sup. .cndot. (but not appreciably HO.sub.2.sup..cndot.) can reduce Fe.sup.3 + to Fe.sup.2 +. These reactions are presented in the following equations: EQU OH.sup..cndot. +H.sub.2 O.sub.2 .fwdarw.H.sub.2 O+HO.sub.2.sup..cndot. EQU HO.sub.2.sup..cndot. .fwdarw.O.sub.2.sup. .cndot. +H EQU HO.sub.2.sup..cndot. +HO.sub.2.sup..cndot. .fwdarw.H.sub.2 O.sub.2 +O.sub.2 EQU HO.sub.2.sup..cndot. +O.sub.2.sup. .cndot. +H.sup.+ .fwdarw.H.sub.2 O.sub.2 +O.sub.2 EQU Fe.sup.3+ +O.sub.2.sup. .cndot. .fwdarw.Fe.sup.2+ +O.sub.2
For organic hydroperoxides (ROOH) which react with metal ions, the generation of free radical species from the reaction of the hydroperoxide with peroxide reactive metal-ions can be formulated as per the following two equations, (where Me symbolizes a peroxide reactive metal-ion), which result in the generation of organic oxy (RO.sup..cndot.) and peroxy (ROO.sup..cndot.) free radicals: EQU ROOH+Me.sup.n+ .fwdarw.RO.sup..cndot. +Me.sup.(n+1)+ +OH.sup.- EQU ROOH+Me.sup.(n+1)+ .fwdarw.ROO.sup..cndot. +Me.sup.n+ +H.sup.+
For organic internal peroxides, the generation of free radical species from the reaction of the peroxide with peroxide reactive metal-ions can be formulated as per the following equation. As used in this specification and in the claims appended hereto, the term organic internal peroxide refers to molecules of the form ROOR', where R and R' are organic moieties which may be identical or different, and wherein both valences of the peroxy (peroxo) --O--O-- moiety are bonded directly to carbon. This definition is to be understood to include endoperoxides. EQU ROOR+Me.sup.n+ .fwdarw.RO.sup..cndot. +RO.sup.- +Me.sup.(n+1)+
It will be recognized by those familiar with the art that free radical reactions are frequently chain reactions, and therefore, once initiated, such reactions can amplify and induce damage far greater in extent than that expected from the number of initiation reactions. It will also be recognized that this amplification factor can sometimes be of major importance.
It will also be recognized by those familiar with the art that the metal-ion mediated free radical generating reactions described above, involve metal-ions which undergo one-electron changes in oxidation state. Metal-ions which undergo two-electron changes in oxidation state (such as tin and lead) can also react with peroxides, but the direct reaction products are generally not free radicals (although it is possible that more distal products may be free radicals). An example of a (non-radical generating) two-electron transfer between a metal-ion and a peroxide is presented in the following equation: EQU ROOH+Me.sup.n+ .fwdarw.RO.sup.- +OH.sup.- +Me.sup.(n+ 2)+
Under certain circumstances, singlet molecular oxygen (.sup.1 O.sub.2) another powerful oxidant, is the reaction product of the reaction between a metal-ion containing compound and a peroxide compound. Spectroscopic evidence for the generation of singlet molecular oxygen (via its 1268 nm emission) has been reported from the reaction of metal-ions with primary, secondary, and tertiary organic peroxides. The mechanism for the generation of singlet oxygen has been most clearly elucidated in the case of secondary peroxides, via the Russell mechanism (J. Amer. Chem. Soc. 79:3872; 1957). Specifically, this consists of the combination of two secondary peroxy free radicals to form an unstable tetroxide, and then the concerted decomposition of the tetroxide to yield an alcohol, a ketone and singlet oxygen (.sup.1 O.sub.2), as per the following equation (the concerted tetroxide decomposition can result in either singlet oxygen and a ground state ketone, or an excited ketone and ground state oxygen): EQU 2R(H)OO.sup..cndot. .fwdarw.R(H)OOOOR(H).fwdarw.R(H)OH+RO+.sup.1 O.sub.2
Singlet oxygen is known to be a powerful oxidant of biological components, but is more selective than the hydroxyl free radical. Singlet oxygen has been shown to oxidize certain amino acids, proteins, unsaturated lipids and reduced pyridine nucleotides, and has also been shown to be markedly cytotoxic.
It is to be understood that the specific mediators and mechanisms of the present invention may be extremely complex and may involve, for example, ferryl and perferryl ions as well as other chemical species. Therefore, the discussions presented herein of specific oxidant species and their mechanisms of generation, such as, for example, hydroxyl free radical and singlet oxygen, are understood to be for illustrative purposes only, and thus the present invention is not to be considered limited or constrained in any way to or by the presented mechanisms and species.
There is clinical documentation for the intravenous administration of dilute peroxide solutions to human patients. Hydrogen peroxide has been used intravenously in humans as a contrast agent for clinical echocardiography. Gaffney et al., (American Journal of Cardiology, 52:607; 1983) injected two milliliters of heparinized 0.2 percent hydrogen peroxide intravenously into 36 patients and noted no detectable ill effects. Wang et al., (Chinese Medical Journal, 92:693; 1979) injected 0.5-1 milliliters of 2-3 percent hydrogen peroxide into 100 patients. Eighty-nine were noted to have no untoward reaction whereas eleven did have side effects which were described as rare and slight. The investigators noted that there was no angina, hemiplegia, or mental disturbance, and that they considered the method to be safe and well tolerated. Oliver and Murphy (The Lancet, 1:432;1920 ) used intravenous hydrogen peroxide in the treatment of influenzal pneumonia with beneficial effect. There is also clinical documentation for the intraarterial use of hydrogen peroxide in human patients (Mallams et al., Prog. Clin. Cancer, 1:137; 1965), but caution has been expressed concerning the intraarterial route (Chasin, et al., Arch. Otolaryng. 85:151; 1967). Benzoyl peroxide has been widely used for approximately two decades on thousands of patients for the topical treatment of acne, with no evidence of serious untoward or deleterious effects. Artemisinin and its derivatives are organic endoperoxides used orally for the treatment of malaria in man, and have been reported to be of relatively low clinical toxicity in the doses utilized (Hien and White, The Lancet, 341:603; 1993).
There have been other therapeutic applications of peroxides in man. Hydrogen peroxide as a three percent aqueous solution has been used for many years as an antiseptic for the skin, and has demonstrated no undue propensity for untoward or deleterious effects. It is also available in a gel formulation for the same purpose. In addition, hydrogen peroxide, calcium peroxide, carbamide peroxide, and sodium peroxyborate, have been used in various oral hygiene preparations such as toothpastes and oral rinses. Carbamide peroxide is available in a gel formulation for the treatment of aphthous ulcers. Carbamide peroxide is also approved for instillation into the ear, in order to loosen impacted cerumen.
It is well known by those familiar with the art that the excessive systemic use of hydrogen peroxide can potentially result in the serious even fatal hazard of oxygen embolus, and that this arises from generating concentrations of oxygen above the oxygen solubility limit of the blood. In order to minimize and prevent this occurrance, it has been proposed that hydrogen peroxide be given in concentrations and quantities, and administered at rates, which preclude the blood oxygen solubility limit from being exceeded (Johnson et al., Br. J. Radiol., 41:749; 1968). The importance of the route, dose, and rate of administration bears emphasis. As a separate matter, some organic peroxide compounds, including benzoyl peroxide, have been reported to be carcinogenic in experiments on animals (Slaga, et al., Science, ;713:1023; 1981, Slaga, et al., pp. 471-484, in Radioprotectors and Anticarcinogens, edited by O. F. Nygaard and M. G. Simic, Academic press, 1983, and J. C. Arcos, et al., Chemical Induction of Cancer, vol. ILIA, pp. 595-604, 1982). The clinical significance of this in humans is unknown particularly regarding benzoyl peroxide, since this compound has a long history of clinical use for the treatment of acne without apparent evidence of carcinogenesis. A number of approved anticancer drugs are known to be carcinogenic, but common sense indicates that, if possible and all other things being equal, noncarcinogenic peroxides are to be preferred.